Aluminum and its alloys (aluminum materials) have been widely used in the automotive industry as lightweight materials. However, compared to their generally excellent corrosion resistance in many applications, the tribological properties of aluminum materials limit their use in certain applications, especially where adhesive wear and abrasive wear properties are required.
A variety of surface treatments have been evaluated to improve wear resistance of aluminum materials. One such widely used treatment involves anodizing the aluminum material to form an anodized hard aluminum oxide coating or layer on the surface such that wear resistance is increased. The anodizing treatment may be followed by a treatment to improve the triological performance of the anodized layer. For example, the anodizing treatment may be followed by formation of a solid lubricating phase in the pores of the anodized layer to reduce the friction coefficient of the anodized layer.
One well known treatment for improving the tribological performance of the anodized oxide layer involves initial formation of MoS3in the pores of the anodized oxide layer. Pursuant to this treatment, the anodized aluminum material is made the anode in an electrolytic cell. The cathode of the cell comprises lead (Pb). The electrolyte comprises an aqueous solution of ammonium thiomolybdenate (NH4)2MoS4 in a concentration of about 0.01 mol/L of the solution with the solution having a pH of 7.1 and temperature of 20 degrees C. A current density of about 0.5 A/dm2 is passed through the cell to produce and deposit MoS3 in the pores of the anodized oxide layer. However, the treated anodized oxide layer must then be thermally heat treated to transform the MoS3in the pores to the desired MoS2 solid lubricant. The thermal heat treatment involves heating the anodized and treated aluminum material at a temperature above 400 degrees C. in nitrogen for a time, such as 6 hours, to effect the transformation of MoS3 to MoS2.
Another treatment for improving the tribological performance of the anodized oxide layer involves formation of MoO2 in the pores of the anodized oxide layer followed by vulcanizing the treated anodized layer at elevated temperature in the presence of H2S gas to convert MoO2 to the desired MoS2 solid lubricant. In this treatment, the anodized aluminum material is made the cathode in an electrolytic cell having an aqueous electrolyte containing molybdenate acid radical (e.g. MO42−). When electrical current is passed between the anode and the cathode of the cell, the radical is reduced to form and deposit a MoO2 synthesis product in the pores of the anodized layer. The treated anodized layer must then be subjected to a vulcanization reaction at an elevated temperature above 500 degrees C. in an atmosphere containing H2S gas to transform the MoO2in the pores to the desired MoS2 solid lubricant.